The present invention relates to a variable frequency oscillator in general, and in particular to a variable frequency oscillator for jamming or interfering with specific TV signals of a cable TV (CATV) system.
Conventionally a phase-locked loop is adapted to a variable frequency oscillator. As is well known, the circuit of a phase-locked loop is provided with a voltage controlled oscillator (VCO), a programmable frequency divider for dividing the output frequency of the VCO, a phase comparator for detecting the frequency and phase differences between the output of the programmable frequency divider and that of a reference oscillator, and a filter circuit for converting the output from the phase comparator to a ripple-free dc voltage and supplying the converted dc voltage to a frequency control terminal of the VCO. When the frequency of this VCO is made variable, means for switching the frequency dividing ratio of the programmable frequency divider is further provided.
With this kind of a conventional variable frequency oscillator, however, the period from the time when the dividing ratio is switched so as to set a new target frequency to the time when the output frequency is stabilized at the new target frequency becomes long. Thus, according to the conventional variable frequency oscillator, the response speed of frequency switching, which is defined by the time period between the setting of the dividing ratio and the stabilization of the output frequency, can not be made high enough. This causes difficulty in obtaining various kinds of frequencies within a certain limited time period (which is frequently several tens of microseconds at most).